The subject invention relates to optical metrology equipment for measuring critical dimensions and feature profiles of isolated and aperiodic structures on semiconductor wafers. The invention is implemented using data obtained from simultaneous multiple angle of incidence measurements as an input to analytical software designed to evaluate surface features via a scatterometry approach.
There is considerable interest in measuring small geometrical structures formed on semiconductor wafers. These structures correspond to physical features of the device including conductive lines, holes, vias and trenches as well as alignment or overlay registration markings. These features are typically too small to be measured with conventional optical microscopes. Accordingly, optical scatterometry techniques have been developed to address this need.
In a conventional optical scatterometry system, a light beam is directed to reflect of a periodic structure. The periodic structure acts as an optical grating, scattering some of the light. The light reflected from the sample is then measured. Some systems measure light diffracted into one or more higher orders. Other systems measure only the specularly reflected light and then deduce the amount of light scattered into higher orders. In any event, the measurements are analyzed using scattering theory, for example, a Rigorous Coupled Wave Analysis, to determine the geometry of the periodic structure.
Rigorous Coupled Wave Theory and other similar techniques rely upon the assumption that the structure which is being inspected is essentially periodic. To match theory to experiment, the diameter of the light beam spot on the sample is typically significantly larger than individual features on the test structure and encompasses many cycles of the grating. Most prior art systems operate wherein the probe light beam spot overlaps at least twenty repeating patterns so that the diffraction analysis will have statistical significance. The results of the analysis represent an average of the geometry illuminated by the probe beam.
In real world semiconductor devices, many (if not most) features are isolated or aperiodic. These isolated structures cannot not evaluated with the grating analysis approaches described above. Accordingly, in order to monitor the geometry of isolated features within the dies on the wafer, manufacturers build test structures on the xe2x80x9cstreetsxe2x80x9d separating the dies. These test structures are periodic but are intended to have the same geometry (e.g. width, shape) as individual features within the die. By measuring the shape of the tests structures, one can gain information about the structure in the dies or overlay registration.
This latter approach has been finding acceptance in the industry. Examples of prior art systems which rely on scatterometry techniques can be found in U.S. Pat. Nos. 5,867,276; 5,963,329; and 5,739,909. These patents describe using both spectrophotometry and spectroscopic ellipsometry to analyze periodic structures and are incorporated herein by reference. See also PCT publication WO 02/065545, incorporated herein by reference which describes using scatterometry techniques to perform overlay metrology.
In addition to multiple wavelength measurements, multiple angle measurements have also been disclosed. In such systems, both the detector and sample are rotated in order to obtain measurements at both multiple angles of incidence and multiple angles of reflection. (See, U.S. Pat. No. 4,710,642)
About fifteen years ago, the assignee herein developed and commercialized a multiple angle of incidence measurement system which did not require tilting the sample or moving the optics. This system is now conventionally known as Beam Profile Reflectometry(copyright) (BPR(copyright)). This and related systems are described in the following U.S. Pat. No. 4,999,014; 5,042,951; 5,181,080; 5,412,473 and 5,596,411, all incorporated herein by reference. The assignee manufactures a commercial device, the Opti-Probe which takes advantage of some of these simultaneous, multiple angle of incidence systems. A summary of all of the metrology devices found in the Opti-Probe can be found in U.S. Pat. No. 6,278,519, incorporated herein by reference.
In the BPR tool, a probe beam from a laser is focused with a strong lens so that the rays within the probe beam strike the sample at multiple angles of incidence. The reflected beam is directed to an array photodetector. The intensity of the reflected beam as a function of radial position within the beam is measured. Each detector element captures not only the specularly reflected light but also the light that has been scattered into that detection angle from all of the incident angles. Thus, the radial positions of the rays in the beam illuminating the detector correspond to different angles of incidence on the sample plus the integrated scattering from all of the angles of incidence contained in the incident beam. The portion of the detector signal related to the specularly reflected light carries information highly influenced by the compositional characteristics of the sample. The portion of the detector signal related to the scattered light carries information influenced more by the physical geometry of the surface.
U.S. Pat. No. 5,042,951 describes an ellipsometric version of the BPR, which, in this disclosure will be referred to as Beam Profile Ellipsometry (BPE). The arrangement of the BPE tool is similar to that described for the BPR tool except that additional polarizers and/or analyzers are provided. In this arrangement, the change in polarization state of the various rays within the probe beam are monitored as a function of angle of incidence. Both the BPR and BPE tools were originally developed for thin film analysis. One advantage of these tools for thin film analysis is that the laser beam could be focused to a small spot size on the sample. In particular, the lens can produce a spot of less than five microns in diameter and preferably on the order of 1 to 2 microns in diameter. This small spot size permitted measurements in very small regions on the semiconductor.
This clear benefit in the thin film measurement field was seen as a detriment in the field of measuring and analyzing gratings with a scatterometry approach. More specifically, a spot size on the order of 1 to 2 microns encompasses less than twenty repeating lines of a conventional test grating. It was felt that such a small sampling of the structure would lead to inaccurate results.
One approach for dealing with this problem was described in U.S. Pat. No. 5,889,593 incorporated herein by reference. This patent describes adding an optical imaging array to the BPR optics which functions to break the coherent light into spatially incoherent light bundles. This forced incoherence produces a much larger spot size, on the order of ten microns in diameter. At this spot size, a suitable number of repeating features will be measured to allow analysis according to a grating theory.
In U.S. Pat. No. 6,429,943 (incorporated by reference), the inventors herein disclosed some alternate approaches for adapting BPR and BPE to measuring periodic gratings. In one approach, the laser probe beam is scanned with respect to the repeating structure to collect sufficient information to analyze the structure as a grating. In another approach, an incoherent light source is used as the probe beam. The incoherent source creates a spot size significantly larger than the laser source and thus could be used to analyze gratings.
Semiconductor manufacturers continually strive to reduce the size of features on a wafer. This size reduction also applies to the width of the streets, typically used as the location for the test structures including overlay registration markings. With narrower streets, the size of the test structures need to be reduced. Ideally, test structures could be developed that were not periodic gratings but closer in form to the actual isolated or aperiodic structures on the dies. Even more desirable would be to develop an approach which would permit measurement of the actual structures within the dies.
With today""s small feature sizes, it has been generally believed that direct accurate measurements of isolated or substantially aperiodic structures could not be performed. An isolated structure would correspond to, for example, a single line, trench, hole or via or a specific alignment mark. Such a structure can have extremely small dimensions (i.e., a single line can have a width of about a tenth of a micron).
In order to optically inspect such small structures, a very small illumination spot is desirable. In the broadband applications such as those discussed above, the probe beam spot size is relatively large, on the order of 50 microns in diameter. If this probe beam was focused on an isolated structure, the portion of the measured signal attributable to the isolated structure would be extremely small. While the spot size of a laser beam is much smaller, it was not envisioned that a enough of a signal could be obtained to measure an isolated feature. Nonetheless, in initial experiments, it has been shown that BPR and BPE techniques using a laser as a probe source can generate meaningful data for isolated structures.
In accordance with this invention, an isolated structure (line, via, etc.) is monitored using an illumination source which is coherent, i.e. supplied by a laser. Such a light source can be focused to a probe beam spot size less than five microns in diameter and preferably less than two microns in diameter. While even this spot size is much larger than the feature of interest, that portion of the measured signal attributable to the feature would be much larger than in a broadband system with a much larger spot size.
The reflected probe beam is monitored with an array detector. As described in the assignee""s patents cited above, when using multiple angle of incidence measurements techniques such as beam profile reflectometry (BPR) and beam profile ellipsometry (BPE), the array detector is used to simultaneously generate information as a function of angle of incidence. The measured data includes a combination of specularly reflected light at specific angles of incidence and scattered light from all of the angles of incidence. In an alternate embodiment, a baffle element is utilized to block the specularly reflected light and maximize the scattered light signal.
The multiple angle of incidence measurements provide information which can be used in a scatterometry analysis. Since the feature is aperiodic, the approach would not take the form of a grating analysis using Rigorous Coupled Wave Theory. Rather, the analysis would have to consider light scattered from the isolated structure such as by using either a boundary integral or volume integral approach.
The subject concept is not limited to investigating a single small feature, but rather, is directed in general to investigating aperiodic structures that cannot be analyzed with a simple grating model.
For example, consider a single structure whose size is larger than the probe beam spot. The measured signal could not be analyzed with a grating model, but if sufficient information is known in advance about the structure, it could be analyzed with a boundary integral approach.
Also consider a periodic structure whose size was smaller than the probe beam spot. The measured signal would be the result of an aperiodic illumination field. Such an aperiodic illumination field could be analyzed with a spatial averaging or a mixing approach (including contributions from both the structure and the surrounding area). One example might be a repeating structure having only 10 lines and where the spot was large enough to cover 10 lines (along an axis perpendicular to the longitudinal lines) as well as areas of equal size outside of the line structure.
The approach can also be used for structures which have some, but incomplete periodicity. For example, consider a line structure having edge profiles that differ over the structure. Such a structure would need to be analyzed as having an aperiodic geometry.
It should be understood that by using the appropriate analysis, one can investigate a variety of both periodic and aperiodic structures without scanning the probe beam to gain additional information. Nonetheless, it should also be understood that scanning the beam with respect to the aperiodic structure can provide additional information. Therefore, it is within the scope of the subject invention to evaluate aperiodic structures by scanning the probe beam with respect to the structure.
In addition to the structures discussed above, the subject approach can also be applied to analysis of the registration of overlying patterns created during lithography steps in semiconductor manufacturing.